Yep, you are going to have to do 400 activities. I didn't grind a lot that season either, so I'm just now at the 370ish mark. It goes a lot faster during the season the weapon was introduced (so get that Witherhoard done now!), because the season pass gives you 4x the progression. Eventually.
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Ehy Hazard =) Yep, other Guardians explained that to me too. I'll take it nice and slowly with time during the next Seasons =)